1. Field of the Invention
This invention relates to apparatus for cooling tubular plastic films extruded from a film blowing head. The invention includes air cooling means consisting of co-axial pipes extending axially through the film blowing head to supply the cooling and inflating air and to withdraw the same from the inflated parison. Vertically spaced apart inner cooling rings are disposed substantially concentrically to the axis of the parison downstream of the annular extrusion die orifice of the blowing head, and the inner cooling rings together with annular members define nozzle gaps and have air guiding surfaces, which face the wall of the parison and are convexly curved in a longitudinal section of the inner cooling rings. During the operation of the apparatus the air guiding surfaces are swept by the cooling air blown out of the nozzle gaps, and the cooling air flows approximately parallel or at an acute angle to the direction of travel of the parison. The cooling means also include outer cooling means, which concentrically surround the extruded parison and comprise an outer cooling ring defining an annular nozzle gap, which is adjacent to and downstream of the annular extrusion die orifice and serves to blow air in a direction which is approximately parallel to the direction of travel of the parison. An apertured baffle plate or shield is provided, which surrounds the parison below its expanding portion adjacent to the down-stream end of the cooling path defined by the inner cooling rings, and the baffle plate or shield in an axial projection on the inflated tubular film covers at least the inner portion of the expanding portion of the parison. The baffle plate or shield has an annular inner edge, which together with the wall of the parison defines an annular gap.
2. Description of the Prior Art
Cooling apparatus has been disclosed in German Pat. No. 3,335,334. By means of that known cooling apparatus the parison can be sufficiently cooled over the length of the cooling path defined by the inner cooling rings, so that after that cooling path the parison can be inflated with a high inflation ratio and damage of the expanding portion of the parison will be avoided. For this reason the cooling apparatus in accordance with the above-mentioned patent can be used to make blown tubular films of high strength, e.g., from polyethylene having a low or higher density because the parison will be longitudinally stretched over the length of the cooling path, in which path the parison has the shape of a bottleneck, and the parison can subsequently be inflated with a high inflation ratio. Owing to the strong cooling action, that high inflation ratio can be achieved adjacent to the neck formed in the parison under its expanding portion. In the cooling apparatus in accordance with the above-mentioned patent, the vertically spaced apart inner cooling rings having convex air guiding surfaces stabilise the parison so effectively adjacent to its bottleneck that it can be swept on the outside with a sharp enveloping cylindrical cooling air jet, which necessarily creates turbulence. A destruction of the inflated parison by this sharp cooling air jet is prevented by the baffle plate or shield which deflects the external cooling air jet radially outwardly and thereby protects the expanding parison. The annular gap formed between the edge of the hole in the baffle plate or shield and the parison is so dimensioned that only an air jet of reduced energy can pass therethrough, to contribute to a further cooling of the expanding parison. The air jet is throttled in the annular gap and simultaneously smoothened so that it will cool the expanding parison without damaging the same.